Transmission system embodying loud-speakers



Oct. 21, 1941. v c, CURL 2,260,170

TRANSMISSION SYSTEM EMBODYING LOUD-SPEAKERS Filed July 18, 1959 3 Sheets-sheet 1 2a F/G. ja L/ ,A AMR A m A m. 'Y "Y 8 /A'A'A' .9 ,F AMR c' F/G. /A

'A AMR AMP.

C AMP.

x 4MB Y /A/l/EA/ TOR H. C. CURL A TTOR/VEV Oct. 21, 1941. H. c. CURL 2,260,170

TRANSMISSION SYSTEM EMBODYING' LOUD-SPEAKERS Filed July 18, 1959 s sheets-sheet 2 'f4 T TORNEV Oct. 21, 1941. H, Q CURL 2,260,170

TRANSMISSION SYSTEM EMBODYING LOUD-SPEAKERS CIRCUIT /A/VE/v TOR H, C. CURL A TTOR/VEV Patented Oct. 21, 1941 UNITED STAT Es PATENT OFFICE 'raNsMlssroN SYSTEM EMBODYING LoUD-srEAKERs.

Herbert C. Curl, Jackson Heights, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a, corporation of New York Application July 18, 1939, Serial No. 285,119

18 Claims. (Cl. 179-1) This invention relates to transmission systems, and more particularly to such systems embodying a plurality of amplifiers supplying signaling energyvto a plurality of remotely spaced loudspeakers. i

In one type of transmission system used heretofore, it has been the practice to so utilize a pair of amplifiers that the outputs thereof are `connected in parallel for supplying signaling energy to a plurality of remotely spaced loudspeakers. In such systems it has been found that a failure of one amplifier may occasion a vfailure of the entire system for the reason that a failure, say, for example, one involving a short circuit in the output of one amplifier, would divert the output of the operative ampliier therethrough so that substantially no signaling energy would Abe supplied to the loud-speakers. This invention contemplates in a transmission system atype of connection in which two amplifiers and a plurality of groups of loud-speakers are so effectively connected that the operation of the system is maintained substantially on a normal basis regardless of a disablement of one amplier or one group of loud-speakersl- It is an object of the invention to obtain increased reliability in the operation of a plurality of amplifiers supplying signaling energy to different groups of loud-speakers in a transmission system. l

It is another object of the invention to mainl tain in such system the normal operation of one group of loud-speakers when the other group of loud-speakers has been disabled.

It is a further object of the invention to provide in such system for the operation of all loudspeakers upon a failure of one of two Vinput amplifiers.

It is a still further object of the invention to provide in such system an optimum impedance match between a signaling input and a plurality of loud-speakers and to maintainA .substantially such impedance match after one-half of the input has been disabled. l

It is still another object of the invention to provide optimum impedance match between a signaling input and a load and to maintain such impedance match within a factor of 3 after onehalf of the load has been disabled.

In a preferred embodiment of the invention, a network comprising a rst pair of opposite arms each of which embodies a resistance, and a second pair of opposite arms each of which embodies a group of loud-speakers has applied to the terminals thereoithe outputs of a pair i A feature of the invention is that while the operation of a transmission system embodying it is maintained regardless of a disablement of one input amplifier or one group of loud-speakers in the manner aforementioned, such system operates in the normal condition without any loss of signaling power.

The invention will be more readily understood from the following description of a transmission system embodying it and by reference to the accompanying drawings in which:

Fig. 1 is a schematic circuit illustrating the invention;

Fig. 1A is a modification that may be incorporated in Fig. 1;

Fig. 2 is a schematic circuit illustrating the normal operation of Fig. 1; and

Figs. 3 through 7 are simplified schematic circuits delineating the operation of Fig. 1 under certain conditions.

Referring to Fig. 1, a plurality of parallel branches 9, each of which embodies a suitable transmitter I0, is applied through a volume control Il to the bridged inputs of ampliers I2 and I3.

Load L1 comprises branches III and I5 and load Lz consists of branches I6 and I1, eachV of which branches includes at least one loudspeaker LS. branch Amay comprise one or more subbranches of various series-parallel combinations in each of which may be connected one or more loudspeakers.

The two sides of each of the branches 9 embody series lresistances 25, 25 which serve the purpose of preventing an accidental short circuit in any one of the transmitters I0 from materially affecting the transmission of the others, as any loss occasioned in these resistances is negligible. Similarly, the inputs of the amplifiers I2 and I3 embody the respective pairs of It is to be understood that eachl the purpose of preventing an accidental short circuit in the inputof one amplifier from materially affecting transmission to .the other, as

any loss to these resistances is negligible.

Also, the branches I4 and I5, and I8 and I1 of the respective loads L1 and La embody similarloud-speakers in branch I 5 to any appreciable extent, depending onthe value of the resistances 28, 28, and vice versa; and also a short circuit occurring in the loud-speakers in branch I8 would not affect the operation oi' the loudspeakers in branch I1 to any appreciable extent, depending on the value of the resistances 28, 28 and vice versa.

Resistance arrangements similar to those mentioned above are disclosed in the patent of H. S. Hamilton et al., No. 1,900,106, granted March 7, 1933.

In accordance with the invention a network 30 is utilized for connecting the outputs of the ani-.- pliiiers |2 and I 3 to the loads L1 and In, and comprises one pair oi' opposite arms, one of which embodies resistance R1 and the other resistance Rz, and another pair of opposite arms one of which embodies the loud-speaker load L1 and the other the loud-speakers load La. The output of the amplier I2 is applied by leads AE and BF to diagonally opposite network terminals I8, I9 while the output of the amplifier I3 is connected by leads CG and DH to diagonally opposite network terminals 2 I, 22. Y

The impedance of each ofthe four arms oi' the network is substantially the same, that is, R1=R2=L1=L2- Theoretically, arms R1 and R2 should be impedance elementswhichsimulate the actual impedance oi' the respective loads L1 and La over a predetermined frequency range. Emplrically, it has been found that resistances equal in value to about the average value oi.' the impedance of the loads L1 and La approximate the theoretical condition to such extent that an operating condition suitable for practical purposes is provided. Also. the internal impedance R of each of the ampliers I2 and I3 is substantially the same and is preferably made to have such value as to provide optimum impedance relation between the ampliiiers and the loud-speaker loads, such relation being that obtaining in a normal circuit, that is, a circuit which does not embody the network. Therefore, the internal impedance of each amplifier is assumed such that R12=R1s=R1=Rz=L1=Im. It is to be 11nv derstood that the equality of R12 and R13 to the other network impedances is not essential to satisfactory operation: and further that such equality is assumed for purposes oi analysis to be explained subsequently, as it approximates the usual operating condition. -Also, the impedances oi all network elements have been taken as pure resistances to simplify the analysis. since in practical appiications the loads L1 and Le may be impedances, in general.

Normal operatimr-Figa 1, 2 and 3 Normally no current iiows in the arms R1 and R: as the network is balanced so that the output ro! the amplifiers I2 and 2,260,170 `series resistances 26,' 28 and 21,21 which serve the loads L1 and La. In such oase the arms R1 and Rz may be considered as non-existent, as no potential is developed thereacross; and both ampliers and loads are effectively connected in series in the manner illustrated in Fig. 2 which will be subsequently explained.

Referring to Fig. 1, output current oi' amplier I2 tends to ow in a two-branch parallel circuit comprising in one branch lead BF, network terminal |9, arm R1', network terminal 22, load L1, network terminal I8 and lead AE, and in the other branch network terminal I9, load Le, network terminal 2|, |8 and lead AE. Output current of amplifier I 3 tends to now in a two-branch parallel circuit comprising in one branch lead DH, network terminal 22, arm R1, network terminal I8, load Le,

network terminal 2| and lead CG, andin the other branch network terminal 22, load L1, network terminal |8, arm R2, network terminal 2| and lead CG.

It is to be noted that the output current o! amplier I2 tends to flow in the network arms in the directions indicated by the solid arrows while the current oi' amplifier I3 tends to ow in the network arms in the directions indicated by the dotted arrows. Thus, the output current of the amplifiers I2 and I3 tends to ilow in the arms R1 and R2 in opposite directions thereby producing across the network terminals YI8 and 22 and across the network terminals I8 and 2| potentials which have the same sign and substantially the same value. Consequently, no current iiows in the arms R1 and R2, Fig. 1. 'I'herefore, a balanced condition is provided in the network.

Inasmuch as no current iiows in the arms R1 and Rz, these armsmay be omitted and the circuit of Fig. l may be simplified to that shown in Fig, 2. In the latter it will be observed that both amplifiers and loads are eietively connected in series. Further, it will be assumed that E is the voltage generated by each amplifier in series with its internal impedance R.

The normal circuit of Figs. 1 and 2 may be further simplified to that illustrated `in Fig. 3 which will now be analyzed.

Inasmuch as the impedances of the ampliiiers, loud-speaker loads, and bridge arms are assiuned to be equal as previously mentioned, then R may represent the impedance of -each amplier and network arm, and the impedance of each loudspeaker load. The reference characters shown in parentheses and associated with certain reference characters in Fig. 3 are ymerely for the purpose of readily identifying corresponding elements in Figs. 1, 2 and 3, and therefore they may be omitted in the following calculations.

'I'he equations for Fig. 3 are:

I.=o, the currentin arm R (R.)

y I is equalizedbetween arm Rz, network terminal Current in the loudspeaker loads is:

I2=g1 the current in load R (L2) (5) It is obvious from Equation 1 that amplifier I2 is operating into a matched impedance load; and similarly from the current delivered by amplier la, that 1s,

that the latter is also working into a matched impedance load, as the current from each amplier E E 27e-IHR (6) in which the first R in the denominator is the internal impedance of each amplifier and the second R therein is the impedance of the elective load for each amplifier.

From the above it is seen that:

(a) No current ows in the network arms, Equations 2 and 3, and therefore Figs. 1 and 2 operate in the normal condition substantially Without any loss of signaling power;

(b) 'I'he same current flows in each loud-speaker load, Equations 4 and 5, and

(c) The impedance of each amplifier is matched to the impedance of each loud-speaker load, Equation 6, that is, optimum impedance match is established between both amplifiers and loads.

Short circuit of one loud-speaker load-Figs. 1 and 4 The occurrence of a short circuit in al1 loudspeakers in one load would have substantially no effect onthe operation of the loud-speakers in the other load, and the current flowing in the operative load would be substantially the same as that owing therein under normal operation.

Assuming a short circuit of the load Lz, or across the network terminals I9 and 2| in Fig. 1, the outp'ut current of amplifier I2 would ow in a circuit comprising lead BF, network terminal I9, arm R1, network terminal 22, load L1, network terminal I8 and lead AE. As the output current of amplifier I2 divides at the. network terminal I9, a portion of this current would also flow in a circuit including network terminal I9, short circuit across the network terminals I9 and 2|, network terminal 2|, arm R1, network terminal I8 and lead AE. The output current of amplier I3 would ow in a circuit comprising lead DH, network terminal 22, load L1, network ter- Imp ,minal I8, arm R2, network terminal 2| and lead CG. As the output of amplier I3 divides at the network terminal 22, a portion thereof would also flow in a circuit including network terminal 22, arm R1, network terminal I9, short circuit across network terminals I9 and 2|, network terminal 2| and lead CG.

Assuming a short circuit of the load L1, or across the network terminals I8 and 22 in Fig. 1, the output current of amplifier I2 would flow in a circuit comprising lead BF, network terminal I9, arm R1, network terminal 22, short circuit across network terminals 22 and I8, network termlnal I8, and lead AE. As the output of amplifier I2 divides at the network terminal I9, a portion of this current would also flow in a circuit including network terminal I9, loadLa, network terminal 2|, arm R2, network terminal I8 and lead AE. The output current of amplifier I3 would flow in a circuit comprising lead DH, network terminal 22, short circuit across network terminals 22 and I8, network terminal I8, arm Rz, network terminal 2| `and lead CG. As, the output current of ampller I3 divides at the net-z work terminal 22, a portion thereof would also flow in' a circuit including network terminal 22, arm R1, network terminal I9, load Lz, network terminal 2| and lead CG.

The circuit of Fig. 1 includingfor example, the short 'circuit of the load L2 mentioned above may, therefore, be represented by the simplified circuit shown in Fig. 4 which will now be analyzed.

It is to be understood that the impedances of the amplifiers, bridge arms and loud-speaker loads may be represented as indicated above with regard to Fig. 3.

The equations for Fig. 4 are:

Then, the current in the operative loud-speaker load R (L1) is:

The current supplied by amplifier I2 is Equation 7, flows in the operative load L1 in Fig. 1 and it is the same as that flowing therein under normal operation, Equations 4 and 5, and

(b) Each amplifier is working into one-third normal load impedance, Equations 8 and 9.

Thus, a short circuit of the load L2 in Fig. 1 would not impair the normal operation of the load L1 in Fig. 1.

It is to be understood that while the above calculations are based on the assumption that the load La in Fig. 1 is' short-circuited and load L1 in Fig. 1 is operative, the same calculations would also apply to the assumed circuit condition in Fig. 1 in which the load L1 is short-circuited and the load La is operative.

Open circuit of one loudl-speaker load-Figs. 1 and 5 'I'he occurrence of an open circuit of one loudspeaker load in Fig. 1 would have substantially no eiTect on the operation of the other loud-speaker.

load, and the amountof current flowing in the operative loud-speaker load would be the same as that flowing therein under normal operation Assuming an open circuit of the loud-speaker load La in Fig. 1, the output current of amplifier I2 would now in a circuit consisting of lead BF,

network terminal I9, arm R1, network terminal 2,2, load L1, network terminal I3 and lead AE; and the output current of amplifier I3 would ow in a circuit comprising lead DH, network terminal 22, load L1, network terminal I3, ann Rz, network terminal 2| and lead CG. Thus, the output current of amplifier I2 would flow in the arm R1 and load L1 in series while the output current of amplifier I3 would flow in the terminal 2I and lead CG. Thus, the output current of amplifier I2 would flow in the load La and arm R1 in series while the output current of amplier I3 would flow in the arm R1 and load La in series. Hence, equal amounts of outy put current oi' each amplifier would flow in each network arm and loud-speaker load.

'I'he circuit of Fig. 1 including, for example. the open circuit oi' the load In mentioned above may, therefore, be represented by the simplified circuit shown in Fig. 5' which will now be ana,- lyzed: v

It is to be understood that the impedances of the amplifiers, bridge arms and loud-speaker loads may be represented as indicated above with regard to Fig. 3.

The equations for Fig. 5 are:

` Solving for I1 and I:

I,+I3=1 the current in loadR (L1) (12) j vious that each amplifieris operating into three ltime's its internal impedance.

From the above it is seen that: (a) 'Ihe current E R Equation 12, flows in the operative load L1 in Fig. 1 and it is the. same as that flowing therein under normal operation, Equations 4 and 5, Aand (b) Each amplifier is working into three times normal load impedance, Equations 10 and ll.

Thus, in Fig. 1 an open circuit of the load L2 would not impair the normal operation of the load L1.

It is to be understood that while the above calculations are based on the assmuption that the load La in Fig. 1 is open-circuited and the load L1 in Fig. 1 is operative, the same calculations would also apply to the assumed circuit condition of Fig. 1 in which the load L1 is opencircuited and the load L2 is operative.

Short circuit or open circuit i'n one amplifier- Figs. 1,-6 and 7 The occurrence of a short or open circuit in either amplifier I2 or I3 in Fig. 1 would cause a 6 decibel loss in all loud-speakers in the loads L1 and L2 relative to normal operation. In such case the output of the operative amplifier would be (1)- divided between the loads L1 and L2 thereby involving a 3 decibel loss, and (2) applied through the arms R1 and R2 to the loads L1 and La, thereby involving a further 3 decibel loss.

Assuming the occurrence of a short circuit in the output of amplifier I2, it is noted that this would be the same as a short circuit across the network terminals I8 and I9. In such case the output of amplifier I3 would flow in a circuit comprising lead DH, network terminal 22, load L1, network terminal I8, arm Rz, network ter-l minal 2I and lead CG. As this current divides at the network terminal 2,2, a portion thereof would also ow in a circuit including network terminal 22, arm R1, network terminal I 9, load L2, network terminal 2I and lead CG. No current would ow in the short circuit across the network terminals I8 and I9 as both of the latter would llave the same potential, or, in other Words, there is no difference of potential thereacross to cause a iiow of current therebetween.

Assuming the occurrence of a short circuit in the output of amplifier I3, it is noted that this would be the same as a short circuit across the network terminals 2l and 22. In such case the output of amplifier I 2 would flow in a circuit embodying lead BF, network terminal I9, arm R1, network terminal 22, load L1, network terminal I8 and lead AE. As this current divides at the network terminal I9, a portion thereof would also flow in a circuit consisting of network terminal I9, load L2, network terminal 2I arm R2, network terminal I8 and lead AE. No current would flow in the short circuit across the network terminals 2| and 22 as both thereof would have the same potential.

Thus, in the case of ashort-circuit disablement of one amplifier, the output of the operative one would be supplied through the network arms R1 and R2 to the loads L1 and L2 thereby involving the 6 decibe?. loss mentioned above. This may be readily seen by referring to Fig. 6 which is a simplified circuit of Fig. 1 when one amplifier say, for example, amplifier I2, is shortcircuited.

Assuming the occurrence of an open circuit in the output of amplifier I2, then the outputoi' La. This may be readily understood by referring to Fig. 7 which is a simplified form of Fig. 1 when one amplifier say, for example, amplifier I2, is open-circuited.

In the case of either a short or an open circuit occurring in the manner aforedescribed, the loss of 6 decibels may be reduced by approximately 3 decibels by short-circuiting each of the network arms and disconnecting the disabled amplifier so as to allow the output of the operative amplifier to be equalized directly between the loads L1 and La. Thus, the net loss in each load would be limited to about 3 decibels (a mismatch loss of 0.5 decibel will occur).

Therefore, it is observed that when one amplifier is disabled by a short or open circuit, the operative amplifier works into the normal load impedance since the four equal impedance arms of the bridge are in series parallel. Consequently, the bridge serves to substantially maintain an impedance match between both loads L1 and La during intervals when both amplifiers are operative and also during intervals when one is disabled and the other is operative.

The circuit of Fig. 1 including say, for example, the short circuit or open circuit of amplifier I2, may be represented by the respective simplified circuits of Figs. 6 and 7 both of which are obviously identical, in so far as a flow of current is concerned, and therefore they may be analyzed together as follows:

It is to be understood that the impedances of the amplifiers, bridge arms and loud-speaker loads may be represented as indicated above with respect to Fig. 3. 4

The mesh equations for Figs. 6 and 7 are:

The current supplied by the amplifier I3 is El 4R is supplied by the operative amplifier to each of the load L1 and Lz in Fig. 1, Equations 13 and 14, and

(b) The internal impedance of the operative amplifier I3 is matched to the effective load, Equation 15, and such impedance match is the same when both amplifiers are operative, Equation 6- Thus, failure of one amplifier output in the manner aforedescribed would not impair the operation of the loads L1 and L1.

It is to be understood that while the above calculations are based on the assumption of a short or open circuit of the amplifier I2 in Fig. 1, the same calculations would also apply to the assumed circuit condition in Fig. 1 in which the short or open circuit occurred in the amplifier I3.

M odifLcatz'on-Fz'gs. 1 and 1A The modification illustrated in Fig. 1A which may be substituted forvtheportion between the dot-dash line X-X and Y -Y shown in Fig. 1 comprises a rst pair of power amplifiers 3I and 32 and a second pair of power amplifiers 33 and 34 whose bridged inputs are supplied to the outputs of the respective input ampliers I2 and I3. The outputs of the amplifiers 3I and 33 are connected in parallel and impressed on one diagonal of the network, and also the outputs of the amplifiers 32 and 34 are connected in parallel and applied to an opposite diagonal of the network. Thus, the input terminals A, B, C and D and the output terminals E, F, G and H of the power amplifiers are connected to the similarly characterized leads in Fig. 1.

The occurrence of a short circuit in the bridged outputs of either power amplifiers 3| and 33, or 32 and 34 would cause a 6 decibel loss relative to normal operation in all loud-speakers in both loads L1 and L2, as the output of the operative pair of power amplifiers would be (l) divided between the loads L1 and L2 thereby involving a 3 decibel loss and (2) impressed on the loads L1 and L2 through the network arms R1 and R2 thereby involving an additional 3 decibel loss. The 6 decibel loss may be reduced to 3 decibels by short-circuiting thearms R1 and R2 and disconnecting the disabled pair of amplifiers so as to allow the output of the operative pair of power amplifiers to be directly applied to the loads L1 bridged amplifiers associated therewithA to the opposite diagonals of the network. As the latter maintains a balanced condition due to the fact l that the potential across the diagonal I8, I9 is substantially the same as that across the diagonal 2 I, 22, the loss occasioned in this manner is limited to 3 decibels. Thus, if amplifier I2 were to be disabled the output of operative amplifier I3 would be applied through bridged amplifiers 33 and 34 to the opposite diagonals of the network; and if amplifier I3 were to be disabled the output of operative amplifier I2 would be applied through bridged amplifiers 3| and 32 to the opposite diagonals of the network. Due to the fact that the network is balanced, the current flow in the two loads L1 and L2 would be equalized.

What is claimed is:

1. In combination, a source of signals, a pair of amplifying means Whose inputs are connected to the signaling source, a pair of load circuits, and means for connecting the outputs of the amplifying means to the load circuits comprising a network having a` pair of opposite arms each of which embodies an impedance and a pair of opposite arms each of which embodies one load circuit, the output of each amplifying means b eing connected to a different diagonal of the network, the-net-work applying the outputs of the amplifying means to the load circuits such that the potentials across opposite diagonals thereof are substantially the same during intervals when both amplifying means are operative and one diagonal has zero potential difference thereacross while the opposite diagonal has the potential of one amplifying means thereacross during intervals when only one amplifying means is operative.

2. In combination, a'source of signals, a pair of amplifying means whose inputs are connected to the signaling source, a pair of load circuits, and means for connecting the outputs of the amplifying means to the load circuits comprising a network having a first pair of opposite arms each of which embodies an impedance and a second pair of opposite arms each of which embodies one load circuit, the output of each amplifying means being applied to a different diagonal of the network, the network applying the outputs of both amplifying means directly to both load circuits during lintervals when both amplifying means are operative and applying the output of one amplifying means through the impedance arms to both load circuits during intervals when the other amplifying means is disabled.

3. The combination according to claim* 2 in which the network applies the output of the operative amplifying means to a two-branch parallel circuit comprising in each branch in series one impedance arm and one load circuit.

4. In combination, a source of signals, a pair of amplifiers connected to the signaling source, a pair of load circuits, and means for providing an optimum impedance match between the amplirler outputs and the load circuits comprising a network having a first pair of opposite arms each of which embodies a resistance and a second pair of opposite arms each of which embodies one load circuit, theoutput of each amplier being connected to a different diagonal of the network, the impedance matching means providing substantially optimum impedance match between the ampliers and the load circuits during an interval when both amplifiers are operative and also during an interval when only one amplifier is operative.

5. In combination, a source of signals, a pair of amplifiers whose inputs are connected to the signaling source, a pair of load circuits, a network.comprising a first pair of opposite arms each of which embodies an impedance and a second pair of opposite arms each of which embodies one load circuit, circuit connections for applying the voutput of each amplifier to a different diagonal of the network, the network supplying the outputs of both amplifiers directly to both load circuits during an interval when both load circuits are operative and supplying the outputs of both amplifiers through the impedance arms to one loadcircuit during an interval when the other load circuit is disabled so as to maintain substantially constant the amount of signaling energy supplied to both load circuits when both thereof are operative and to the one load circuit when the other load circuit is disabled.

6. In combination, a source oi' signals, a pair of amplifiers whose inputs are connected thereto, a pair of load circuits, and means for connecting the amplifier outputs to the load circuits comprising a network having a first pair of opposite arms each of which embodies an impedance and a second pair of opposite arms each of which embodies one load circuit each amplifier output being applied to a different diagonal of the network, the network supplying both amplifier outputs directly to both load circuitsduring intervals when both load circuits are operative and supplying both amplifier outputs through the impedance arms to Aone load circuit during intervals when the other load circuit is disabled thereby equalizing the potentials across the different network diagonals when both load circuits are operative and also when one load circuit is operative and the other is disabled.

'7. In combination, a source of signals, a first input amplifier, a second input amplifier, the inputs of the first and second input amplifiers being connected to the signaling source, a first pair of power amplifiers whose inputs are connected in parallel to the output of the first input amplifier, a second pair of power amplifiers whose inputs are connected in parallel to the output of the second input amplifier, the outputs of the first pair of power amplifiers being bridged with the outputs of the second pair of power amplifiers so as to provide two bridged power outputs each of which includes one amplifier of each pair of power amplifiers, a pair of load circuits, `and means for connecting the two bridged power outputs to both load circuits comprising a network having a first pair of opposite arms each of which embodies an impedance and a second pair of opposite arms each of which embodies oneload circuit, each bridged power output being applied to a different diagonal of the network, the network directly applying both bridged power outputs to both load circuits during intervals when both bridged power outputs are operative and applying one bridged power output through .both impedance arms to both load circuits during intervals when the other bridged power output is disabled.

8. The combination according to claim 7 in which the network applies directly both bridged power outputs to both load circuits during an interval when both input amplifiers are operative and also during an interval when one input'amplifier is operative and one input amplifier is disabled.

9. In combination, a pair of amplifying means, a pair of load circuits, and means to connect the outputs of said amplifying means to said load circuits, comprising a network having a first pair of lopposite arms each of which embodies an impedance and a second pair of opposite arms each of which embodies one of said load circuits, and the output, of each amplifying means connected to dierent terminals of said network.

10. In combination, a first input amplifier.' a. first pair of power amplifiers, circuit means to connect in parallel the inputs of said first pair oi' power amplifiers to the output of said rst input amplifier, a second input amplifier, a second pair of power amplifiers, circuit means to connect in parallel the inputs of said second pair of power amplifiers to the output of said second input amplifier, circuit means to connect the outputs of said first and second pairs of power amplifiers to provide two independent bridging outputs each of which embodies one ampliiler of each'of said first and second pairs of power ampliiiers, a pair of load circuits, and-circuit means to connect said two independent bridging outputs to said load circuits, comprising a network having a rst pair of opposite arms each of which embodies an impedance and a second pair .of opposite arms each of which embodies one of said load circuits, and each independent bridging output connected to different terminals of said network. v

11. In co bination, a pair of amplifying means, a pair of lad circuits, and means to apply the outputs of said amplifying means to said load circuits such that said outputs and said load circuits are eiectively connected in series in one circuit.

12. In combination, a pair of amplifying means,

a pair of load circuits, and means to connect effectively the outputs of said amplifying means and said load circuits in series, comprising a network having a first pair of opposite arms each of which embodies an impedance` and a second pair of opposite arms each of which embodies one of said load circuits, and they output of each amplifying means connected to different termi nals of the network and the potential across the terminals of each impedance arm being substantially equal and yhaving the same sign so that substantially no amplified current flows therein.

13. In combination, a pair of amplifying means each of whose outputs possesses substantially the same impedance, a pair of load circuits each of which possessessubstantially the same impedance, the impedances of each output and each load circuit being substantially equal, and means to provide substantially optimum impedance match between said outputs and said load circuits, comprising a network having a rst pair of opposite arms each o f which embodies an impedance whose amount equals substantially each output or load circuit impedance and a second pair of opposite arms each of which embodies one of said load circuits, and each output connected to diiferent terminals of said network so that substantially the same amount of amplified current flows in each load circuit.

14. In combination, a pair of amplifying means,

a pair of load circuits, and means to apply eiiectively the outputs of said pair of amplifying means to said pair of load circuits such that a certain amount of ampliiied signaling current flows in each of said pair of load circuits when .both thereof are operative and said certain amount of amplied signaling current flows in an operative load circuit when the other load circuit is disabled.

15. In the combination according to claim 13 in which said matching means connects eiectively the output of each 'amplifying means substantially to one-third optimum load impedance when one load circuit is operative and the other load circuit is short-circuited.

16. In the combination according to claim 13 in which said matching means connects effectively the output of each amplifying means substantially to three times optimum load impedance when one load circuit is operative and the other load circuit is open-circuited.

17. In the combination according to claim 13 in which said matching means effectively connects the output of one operative amplifying means substantially to optimum load impedance when the outputof the other amplifying means is either short-circuited or open-circuited.

`1 8. In combination, a source of signals, a pair` of amplifying means connected to said signaling source, a pair of load circuits, and means for effectively connecting the outputs of both amplifying means to both load circuits and maintaining such connection during an interval when one amplier is disabled and also during an interval when one load circuit is disabled, said connecting means comprising a network having a iirst pair of opposite arms each of which embodies an impedance and a second pair of opposite arms each of which embodies one load circuit, and circuit means to apply each outputto a different diagonal of said network.

HERBERT C. CURL. 

